If I do getLastInsertId() immediately after a save(), it works, but otherwise it does not. This is demonstrated in my controller:
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CakePHP has two methods for getting the last inserted id: Model::getLastInsertID() and Model::getInsertID().
Actually these methods are identical so it really doesn't matter which method you use.
echo $this->ModelName->getInsertID();
echo $this->ModelName->getLastInsertID();
This methods can be found in cake/libs/model/model.php on line 2768